This invention relates to earth engineering, especially relative to short aggregate pier implementations. Specifically, this invention relates to methods and apparatus for improving the feasibility of short aggregate piers, improving the strength and load-bearing characteristics of the soil matrix in the vicinity of short aggregate piers, reducing the costs of constructing short aggregate piers, improving the construction of short aggregate piers, and employing short aggregate piers in engineering applications involving settlement control bearing capacity improvement, lateral load resistance, landslide control and uplift anchoring.
It is known to strengthen otherwise inadequate load-bearing capacity of soil by formation of short aggregate piers, such as those disclosed in U.S. Pat. No. 5,249,892, the subject matter of which is incorporated in its entirety herein by reference. Generally, short aggregate piers are constructed in situ by individually compacting a series of thin lifts or layers of aggregate within a cavity formed in the soil. When each lift is compacted, vertical compaction forces are transferred through the aggregate vertically and laterally outward to the surrounding soil. The pier resulting from a vertical xe2x80x9cstackxe2x80x9d of lifts, each compacted before the next lift is formed and each including aggregate elements which are not cohesive, is characterized by the ability to transfer a relatively large portion of the load outward and laterally into the adjacent, prestressed soil. Short aggregate piers have been recognized in the civil engineering field as revolutionary, partly because they provide for increased load-bearing capacity in soil environments which would otherwise tend to make construction of adequate foundations expensive or unfeasible.
Because short aggregate piers are a relatively recent development, much effort lately has been expended towards improving their feasibility, reducing their cost and expanding their field of use and improving their construction. The present invention provides several unique and novel techniques, which include novel methods and the use of novel apparatus that provide the advantages of improving the feasibility of short aggregate piers, reducing their cost, expanding their field of use and/or improving their construction.
Feasibility of short aggregate piers in certain soil environments is limited by the load-bearing characteristics of matrix soils. These characteristics include the soil shear strength, compression characteristics, compactibility, density and permeability or affinity for water. For example, it has been recognized that, especially in loose or soft soil environments, undesirable degrees of settlement may occur when the bearing load is applied to short aggregate piers constructed according to known techniques. It has also been recognized that in known short aggregate pier constructions, very soft subsoils may result in excessive material construction costs due, for example, to the absorption of a significant amount of aggregate stone into the bottom bulb of the short aggregate pier during the tamping or compaction step of construction. These shortcomings tend to limit the feasibility of known short aggregate pier techniques. Thus, it would be desirable to provide techniques which tend to favorably influence or improve the load-bearing characteristics of matrix soils and/or matrix soils in combination with short aggregate piers. It would further be desirable to provide for construction techniques for short aggregate piers which would tend to expand the feasibility of short aggregate piers across a wide range of soil compositions.
Another problem recognized since the development of heretofore known short aggregate pier techniques relates to xe2x80x9cplasticxe2x80x9d or xe2x80x9cheavingxe2x80x9d soil environments. These types of soil matrices are characterized by a high-volume change potential due to the absorption of water. Such soil compositions tend to create the potential for undesirable uplift forces on short aggregate piers and on surrounding matrix soils. Uplift forces are undesirable because supported structural members such as footings, mats, beams and slabs will tend to move upward, causing structural distress and/or cosmetic damage. It would therefore be desirable to provide short aggregate pier techniques which reduce the potential for uplift forces in plastic or heaving soil environments. Yet another limit on the feasibility of known short aggregate pier techniques relates to liquefiable soil environments (i.e. soils which tend to liquefy when subject to sufficient dynamic stress). Typically, engineering codes may require special aggregate drain gradation in structures formed in liquefiable soil environments. Present aggregate drain structures are not constructed in thin lifts and are, as a result, not as efficient in providing settlement control or soil liquefaction control as can be provided using known short aggregate technique with compaction of each lift (the short aggregate pier technique). It would therefore be desirable to provide short aggregate pier techniques which provide the ability to meet aggregate drain gradation requirements.
Another problem recognized with known short aggregate pier construction techniques is that in soft or unstable soil environments, the pier cavity may tend to distort, cave-in, or become otherwise damaged as the pier is formed in situ. It would therefore be desirable to provide for short aggregate pier construction techniques which reduce the potential for damage to the pier cavity during pier construction.
Yet another problem recognized with known short aggregate pier techniques is that the impact loading typically applied to compact each lift during construction of an short aggregate pier may be problematic in areas which are in close proximity to sensitive structures (i.e. older buildings) or buried objects, such as pipes, culverts or conduits. It would therefore be desirable to provide short aggregate pier construction techniques which do not present a danger to damaging nearby sensitive structures.
As short aggregate piers are desirable, in part, because they are economical, it is desirable to provide for construction techniques which reduce the cost of short aggregate piers compared to known construction techniques. It is also desirable to provide construction techniques which maintain the integrity of the matrix soils susceptible to damage during construction. Still further, because of the advantages recognized in known short aggregate pier techniques, it would be desirable to expand the field of use of short aggregate piers to earth engineering areas that include, for example, global stabilization, lateral load resistance, landslide control and uplift control. Finally, it would be desirable to provide methods and apparatus for obtaining stress distribution and other data from short aggregate piers which have been constructed.
The aforementioned problems and desired advantages are realized by the present invention, which provides several novel and unique methods and apparatus that improve the feasibility of short aggregate piers, improve the strength and load-bearing characteristics of the soil matrix in the vicinity of short aggregate piers, reduce the costs of constructing short aggregate piers, improve the construction of short aggregate piers, and employ short aggregate piers in engineering applications involving settlement control, bearing capacity improvement, lateral load resistance, landslide control and uplift anchoring.
A. Short-Aggregate Pier Construction Techniques for Improving Soil Characteristics.
One aspect of the present invention relates to methods for constructing short aggregate piers to improve the load-bearing characteristics of the soil and to short aggregate pier constructions made by such methods. These techniques include: 1) methods and apparatus for preloading soils and short aggregate piers; 2) methods for providing chemical additives to short aggregate pier constructions; and 3) methods for employing mesh reinforcement in short aggregate pier constructions and short aggregate piers constructed by those methods. These techniques make short aggregate pier implementations more feasible in problematic soil environments.
1. Construction Techniques Using Preloading or Prestraining.
a) Prestraining or Preloading of short aggregate piers.
Short aggregate piers are compressible. When a load is applied, they tend to slightly bulge out, and also to compress vertically within the aggregate as the aggregate densifies. There is also vertical deformation as the adjacent matrix soil strains to mobilize side friction for load resistance through vertical shear resistance. By xe2x80x9cprestrainingxe2x80x9d a short aggregate pier, much of the deformation that occurs is irreversible, and is permanently eliminated. This is because of inelastic deformation within the short aggregate pier itself (densification of aggregate), and inelastic prestraining of the matrix soils adjacent to and underneath the short aggregate pier. The new effect is xe2x80x9cstiffeningxe2x80x9d the pier, so that for the same deformation to occur as occurred the first time it was xe2x80x98prestrainedxe2x80x99, a larger load can be applied. Conversely, the short same load applied the second time will result in less deformation because of this prestraining or preloading, and stiffening of the pier.
This has been proven in experiments with full-scale short aggregate piers. A number of load tests have been performed on piers where, after the maximum required load was applied, it was removed and a second load was applied to the same magnitude as the maximum previous load. In some cases the second loading was applied immediately after the first load sequence. In other cases, the second load application was applied one day to several days after the first loading. Deformations were measured for all load increments, both the original cycle and for the second, or prestrain cycle. Typically, about 50% of the total deformation was eliminated when the second load was applied. An additional load that caused the short same deformation which occurred from the first maximum load application was on the order of 150% as great as the magnitude of the first maximum loading.
Likewise, the prestraining may be applied by using an hydraulic jack or other device and jacking against a heavy reaction, such as a heavy piece of equipment, heavy vehicle, or dead weights. It may also be applied by a special apparatus inserted into the short aggregate Pier cavity. This apparatus obtains necessary reaction force by intimate contact with matrix soils adjacent to the cavity, and application of lateral force to the cavity walls while providing a set of shear plates in contact with the soil. It may also be applied by dynamic force such as a controlled explosion or by dropping weights.
b) Prestressing and Prestraining of Piles, Drilled Piers (Caissons), xe2x80x9cStone Columnsxe2x80x9d, And Other Foundation Elements.
A similar second or multiple re-application of load will likewise result in prestraining the matrix soil and preloading the foundation element-matrix soil system. For relatively incompressible materials such as steel and concrete, there will be no significant inelastic permanent deformation in the element itself. However, within the adjacent matrix soil and underlying matrix soil such a preloading will cause inelastic deformation and prestraining, which will result in either reducing future deformations under identical loads or in allowing increased loads to be applied to result in the short same deformation magnitude as originally occurred.
These prestressing and prestraining relationships discussed above for piles, drilled piers, and other incompressible foundation elements have been proven in a number of special sort aggregate pier load tests where xe2x80x98tell-talesxe2x80x99 were installed. The xe2x80x98tell-talesxe2x80x99 (see # 14 below) are steel plates and rod devices, placed at the bottom of the tested pier to determine the magnitude of deflection occurring be low the pier bottom. The difference between magnitude of deflection at pier bottom and magnitude of deflection at pier top is equal to compression within the pier upon loading. During the re-cycle test it can be shown that the magnitude of total deflection (deflection measured at short aggregate pier top) exceeds the magnitude of pier compression deformation caused by compression of the pier itself (equal to the magnitude of deflection measured at short aggregate pier top minus magnitude measured). The difference between total deflection measured in the re-cycle test and pier compression deformation represents the reduced deflection which would have occurred if the short aggregate pier were incompressible, as essentially is concrete and steel (in comparison with soil). This shows that prestraining and preloading other foundation systems will result in less vertical deflection occurring under the short same load when a second load is applied, or more load is available to produce the same amount of deflection for a second load application. The prestraining application may be applied by jacking against a heavy reaction or by a special apparatus, both as described in (a) above. It may also be applied by dynamic force, such as a controlled explosion or a dropping weight.
2. Construction Techniques Using Chemical Additives.
Chemical additives can be added to the aggregate in short aggregate piers, whether by mixing with the aggregate, or by placing in lifts between aggregate lifts, or by placing in the cavity at the periphery of the aggregate. The chemical additives will react and combine with both the aggregate and the adjacent matrix soils. The reaction of chemical to soils is generally understood for most chemical additives that are to be used with this method, including cement, hydrated lime, quicklime, and flyash. For example, quicklime may be added to the aggregate pier to improve the shear strength and compressibility characteristics of the adjacent matrix soils. The end result is an improvement in soils strength and in the reinforcement efficiency of the chemically-aided short aggregate pier system.
3. Short Aggregate Pier Construction Techniques Using Mesh or Geofabric Reinforcement.
Geogrids or synthetic or metal mesh or geofabric may be used placed horizontally between aggregate lifts to increase the short aggregate pier stiffness, or vertically within the perimeter of the short aggregate pier to reduce bulging of the pier in very soft, compressible matrix soil materials. Use of geogrids or synthetic or metal mesh or geofabric materials within short aggregate piers will result in efficient and effective tension resistance and resulting effective increase in lateral shear resistance and pier stiffening. Geogrids have been used in an experimental short aggregate pier modulus load test. The result showed a restraint in bulging and lateral displacement of the pier when the pier was wrapped with geogrid placed vertically along the perimeter of the short aggregate pier.
B. Short-Aggregate Pier Construction Techniques for Expanding The Feasibility of Short Aggregate Piers.
Another aspect of the present invention relates to methods for constructing short aggregate piers to expand the feasibility of short aggregate pier constructions. These techniques include: 1) short aggregate pier construction techniques using selective gradations of aggregate; 2) short aggregate pier construction techniques for reducing friction; 3) short aggregate pier construction techniques for controlling liquefaction of soil; 4) Non-impact short aggregate pier construction techniques; and 5) the use of sand, soil, low slump or no slump, xe2x80x9croller concrete,xe2x80x9d or other indigenous materials in short aggregate pier constructions. These techniques employing short aggregate pier implementations expand the feasibility of short aggregate piers to a broader range of soil environments than prior art techniques.
1. Short Aggregate Pier Construction Techniques Using Selective Gradations of Aggregate.
The present invention relies on the discovery by the present inventor that constructing a short aggregate pier to maximize effectiveness as a soil reinforcement element requires the use of different gradations of aggregate-both for use below the water table and for construction of the bottom bulb which is part of the short aggregate pier. xe2x80x9cWashedxe2x80x9d or xe2x80x9ccleanxe2x80x9d stone, such as 1 to 1.5 inch maximum size graded stone in which the fine sands, silt and clay fractions have been removed or are limited to small percentages, are typically used for the bottom lift constructions in forming a xe2x80x9cbottom bulb.xe2x80x9d If the subsoils are very soft, the 1 to 1.5 inch washed stone which has been used in prior art techniques to create the bottom bulb, results in a bulb which is too deep because of the excessive energy of the tamper in relation to the bearing capacity of the very soft soils. For this situation, a larger stone, such as a 3 or 4 inch minus stone is substituted for the 1 to 1.5 inch diameter stone. The result is a shorter bulb and a dampening out of the tamping energy by the larger stones. Another technique that may be used is to mix two or more gradations of stone together to produce a more desired gradation of stone. An example may be #57 stone (1 to 1.5 inch maximum sized, washed stone) mixed with #68 stone. Yet another techniques is to layer different gradations of stone on one another. One may begin with the larger, 3 or 4 (or 6) inch diameter stone. After one or more lifts of this stone, one may add the 1 to 1.5 inch diameter washed stone. After one or more lifts of this stone, one may add highway well-graded base course stone.
2. Short Aggregate Pier Construction Techniques For Reducing Side Friction.
Normally, a short aggregate pier is intended to generate as much side friction as possible with the adjacent matrix soils to assist in resisting vertical compressive forces or vertical uplift shear forces. However, when soils are of high volume change potential (also knows as xe2x80x9cplasticxe2x80x9d or as xe2x80x9cheaving soils.xe2x80x9d), the active upper zone of these soils, which is the zone that changes volume because of moisture change, tends to pull up or lift the aggregate pier. For this portion of the pier (not for the underlying, lower portion of the pier), it is advantageous to reduce the side friction between the short aggregate pier and the matrix soil. According to the present invention, this can be accomplished with the use of space-age polymers, or with BENTONITE or other lubrication materials. The invention also contemplates the use of liners made of cardboard, plastic, or metal to reduce side friction within the active zone.
3. Short Aggregate Pier Construction Techniques For to Controlling Liquefaction.
Short aggregate piers can be constructed to meet aggregate drain gradation requirements. They may also be constructed so that a portion of the short aggregate pier within liquefiable soils, meets the aggregate drain gradation requirements while other portions that are adjacent to soils that are not liquefiable, do not have to meet such gradation requirements. What makes this type of aggregate drain unique is that it is a short aggregate pier xe2x80x9caggregate drainxe2x80x9d, and therefore meets all requirements of a short aggregate pierxe2x80x94i.e., constructed within a cavity, constructed with granular materials placed in thin lifts, and constructed by tamping the lifts to densify them and to prestrain and prestress adjacent and underlying soils.
The use of special gradation to produce liquefaction control is a known method of controlling liquefaction. However, to date, no one has constructed short aggregate piers with thin lifts of aggregate compacted on each liftxe2x80x94and meeting aggregate drain gradation requirements Such aggregate drain short aggregate piers will have the advantage of greater strength, greater matrix soil strength, and less compressibility than normal aggregate drains that are either not compacted at all, or are compacted only in thick lifts (generally 6 to 10 feet thick or more). It has been shown that increased aggregate drain densification results in more effective aggregate drains. Such will be provided with short aggregate piers incorporating aggregate drain gradation.
4. Non-impact Short Aggregate Pier Construction Techniques.
Known methods of making short aggregate piers use dynamic impact forces produced by impact ramming action. When sensitive structures or objects such as buried conduits, buried culverts or piping, old historic buildings, etc. are either underground, partially underground, or on top of the ground, and in the proximity of the short aggregate piers, then static piers may be constructed, or static piers with a vibrating tamper can be constructed. The short aggregate pier thus constructed will generally not be as effective in controlling settlements or controlling uplift or lateral forces as will the impact-constructed short aggregate pier of the same diameter and shaft length. However, this modified technique will protect sensitive objects in the vicinity of the short aggregate pier construction location. Total capacity of the system can be made comparable to the impact short aggregate pier system, even though the capacity per square foot of planar area is less, This can be accomplished by: 1) increasing the number of piers supporting the same structure: 2) increasing the average diameter of each pier; 3) reducing the thickness of the pier lifts; 4) using geogrid or mesh; or 5) a combination of these methods.
Applying a limited amplitude vibrating source to the static load-applying tamper or tamper shaft will generally make the short aggregate pier stiffer than it would be with static load alone, but not as stiff as it would be with dynamic impact forces and ramming action. Lifts can be done in such a way that resulting stresse s which could damage adjacent sensitive structures are reduced compared with those produced from dynamic impact loads in order that adjacent sensitive structures are not damaged.
5. The Use of Sand, Soil,xe2x80x9cRoller Concretexe2x80x9d and Other Indigenous Materials In Short Aggregate Pier Construction.
In addition to stone aggregate, sand, soil, chemically-treated soils or xe2x80x9croller concretexe2x80x9d may be used as building materials for short aggregate piers. The sand or other soils may be imported from commercial sources, or they may be indigenous to the construction site or immediate area. xe2x80x9cRoller concretexe2x80x9d may be manufactured on site or brought to the site from a mixing plant. The basic steps in making a short aggregate pier, of creating a cavity, densifying and prestraining/prestressing soils at the bottom of the cavity, and building up the short aggregate pier shaft by placing materials in thin lifts and densifying them, still remains. The difference is that these materials need not be stone aggregates. Laboratory and small-scale field experiments have shown that sand, chemically-treated sand, and chemically-treated soils (silts and clays), and soils with synthetic inclusions mixed within the soils or placed in layers, can be effectively used as short aggregate pier building materials. The chemicals most often used in the experiments, and considered to be the most practicable for production of short aggregate piers, are cement, hydrated lime, quicklime, and flyash. xe2x80x9cRoller concrete,xe2x80x9d being low moisture content, low slump or no slump concrete, behaves in a similar fashion to soil, and may be placed and compacted in thin lifts.
C. Short-Aggregate Pier Construction Techniques for Reducing The Cost of Short Aggregate Piers.
Yet another aspect of the present invention relates to methods for reducing the cost of short aggregate piers or improving their construction. These techniques include: 1) short aggregate pier construction techniques using variable diameter lifts; 2) short aggregate pier construction techniques using recycled, interlocking concrete aggregates; 3) short aggregate pier construction techniques employing casings to protect the pier cavity; and 4) short aggregate pier construction is techniques employing load sensors, load cells, or pressure cells within, adjacent to, or below the short aggregate pier structure. These techniques reduce the cost or otherwise improve the construction of short aggregate piers.
1. Short Aggregate Pier Construction Techniques Using Variable Diameter Lifts.
Known short aggregate piers have been made with a single diameter or a similar rectangular cross section shape. By designing and constructing piers with two or more diameters, or two or more rectangular cross section areas, the smaller diameter or cross section area being within the lower portion of the pier, and the larger diameter(s) or cross section area above, will allow more efficient use of stone and tamping energy. Stresses from structural loading are known to dissipate or lessen with depth within short aggregate pier elements. By creating short aggregate piers with smaller diameters in lower portions, (or smaller rectangular cross section areas) one can take advantage of the lower magnitude of stresses and save time and costs related to: (1) aggregate materials, (2) to drilling, and (3) to required compaction efforts.
2. Short Aggregate Pier Construction Techniques Employing Recycled or Interlocking Concrete Aggregate.
A practicable substitute for commercial, graded aggregate in short aggregate pier construction is to use recycled, and re-crushed graded concrete. The advantages of this include: 1) the cost of re-cycled concrete is normally significantly less than processed commercial graded aggregate, in cost per ton of material; 2) the edges of re-cycled concrete are often sharp, and often sharper than corresponding edges of commercial graded aggregate. This is particularly true when commercial aggregate source is a glacial or alluvial stone with some rounded surfaces. Sharper edges results in greater frictional shear and more effective interlocking between particles. This in turn, results in stronger short aggregate piers; 3) another advantage recognized by the present invention is that the chemicals contained in the original concrete typically including Portland cement and lime, are still partially active after many years of concrete usage. These chemicals improve the aggregate by both forming a binder or cementing action within the aggregate itself, and also by combining chemically with the adjacent soils.
3. Short Aggregate Pier Construction Techniques Employing Casings to Protect the Pier Cavity.
Temporary casings may be used to keep short aggregate pier cavities open during construction of the short aggregate pier. A unique requirement of the use of casing is that the casing must be lifted in short vertical increments essentially equal to the lift thicknesses of the placed aggregate, This allows for lateral soil prestressing, prestraining, and densification, during compaction of each lift. This lateral prestressing and prestraining would not be possible if the liners were not lifted during compaction of each lift. Additionally, lifting the liners prevents the liners from containing densified aggregate xe2x80x9cplugsxe2x80x9d which occur if aggregate is compacted within the temporary casings. These xe2x80x9cplugsxe2x80x9d either cannot be removed, or are very difficult to remove because of the high lateral force on the liners and the high coefficient of friction between the aggregate and the liner. The casing may be lifted by special lifting apparatus such as a crane, a forklift, an excavator, or other piece of construction equipment.
4. Short Aggregate Pier Construction Techniques Employing Load Sensors or Pressure Cells Within the Pier Structure.
In accordance with the present invention, load cells or load sensors may be placed within short aggregate piles to determine stress levels within different depths of a short aggregate pier. These data can be used to evaluate stress distribution within short aggregate piers, which in turn can be used to better understand how short aggregate piers work, and proper designation of pier capacities. In addition, pressure cells may be installed within portions of short aggregate piers. These pressure cells may be activated, causing pressure to expand the cells and push upward and downward on portions of the short aggregate piers. This will cause movement or deformation of the short aggregate piers. Measurement of these movements provides information concerning stiffness of the pier and capacity of the pier which can be used in design of the pier. Tell-tales, consisting of a bottom member fixed to vertical members, can be emplaced within short aggregate piers to measure displacements or deformations occurring below the telltales during load testing or during actual structural loadings. This can be used to better estimate and understand the load-deformation characteristics of short aggregate piers.
5. Techniques For Testing Characteristics of Short Aggregate Piers.
Also in accordance with the present invention, techniques are provided for testing characteristics, such as stiffness, of short aggregate piers, also known as a GEOPIER. In a preferred embodiment of the invention, a load transmitting element (normally the tamper assembly) is provided in tandem with a calibrated pressure or load measuring device on the pier to determine magnitude of applied load. A deflection measuring device is provided on load transmitting element or on other reference location integral with top of short aggregate pier. When a load is applied through the load transmitting element to the top of pier, the load is measured with the load measuring device and the downward deflection of the pier is measured with the deflection measuring device. The pier stiffness may be calculated from the measured load and deflection.
The unique aspects of this feature of the invention provide for verification of characteristics, such as the stiffness modulus, of short aggregate piers, in situ. The invention provides the ability to combine a deflection measurement data with load or pressure measurement data to verify the modulus of the short aggregate pier immediately after construction In the case of inadequate or unacceptable stiffness, stiffness can be increased by re-application of densification energy, including possible partial re-drilling and re-building of pier. Alternatively, insufficient pier stiffness may be taken into account in the design, and the pier may remain as it is, with less capacity than originally designed.
D. Using Short Aggregate Piers For Lateral Load Resistance And As Uplift Anchors
In accordance with yet another aspect of the invention, short aggregate piers its are employed in applications for global soil stabilization, lateral load resistance and landslide control and as uplift anchors.
1. Global Stabilization, Lateral Load Resistance and Landslide Control.
Full-scale field shear tests within short aggregate piers show that the shear strength available within short aggregate piers is unusually high with friction angles above 50 degrees with highway base course stone, and 48 degrees for washed 1 to 1.5 inch diameter stone. This very high shear strength provides the capacity of the short aggregate piers to resist lateral forces and to resist shear forces. As a result, the short aggregate pier is very efficient in providing global stability to resist a global or internal shear failure. Short aggregate piers are also efficient in providing resistance to landslides and to lateral load restraints such as when foundations supported by short aggregate piers are loaded laterally. The coefficient of friction in this latter case is essentially equal to the tangent of the friction angle of the aggregate pier itself. Another factor which contributes to the lateral load resistance and global stability provided by the short aggregate piers is the stress concentration which occurs within the piers.
2. Uplift Anchors.
The short aggregate pier, when equipped with axe2x80x9charnessxe2x80x9d or device for transferring load from the pier bottom to the structure undergoing uplift forces, acts as a very efficient uplift anchor. The device or harness consists basically of a bottom plate or series of plates, fixed with vertical bars or tubes that transfers forces to the footing, slab, or beam that needs uplift resistance. The short aggregate pier, with its high coefficient of friction between itself and the matrix soil, provides an exceptionally efficient uplift anchor per foot of depth. Another factor contributing to the effective uplift force resistance of the short aggregate pier is the build-up of lateral soil stresses which occur during the short aggregate pier installation. These anchors may be permanent or temporary, depending on usage. A number of experimental uplift load tests have been performed which confirm the high uplift capacity provided by the short aggregate pier system equipped with stress transfer mechanisms.
Other advantages, novel features, and the further scope of applicability of the present invention will be set forth in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.